In systems for wireless transmission of information using electromagnetic signals, for example cellular telephony, the area which is covered by the system is often divided into smaller areas, so-called cells. In each cell there is a centrally located so-called base station, with which each user of the system in the cell communicates. It is necessary that the antennas of the base stations are installed in positions which are high above ground, and thus clearly visible in cities, for example on rooftops, walls, etc. For aesthetic reasons this, of course, creates a requirement for making the base stations as compact as possible.
Another requirement on the base stations is for them to use as little energy as possible. So far, to a great extent, base stations have been used which are essentially omnidirectional, in other words they transmit equal amounts of energy in all directions. Modern technology, however, permits the building of so-called "steerable antennas", which means that the beam, or lobe, of the antenna is directed only in the direction where there is a subscriber at the moment. The beam can then be controlled to follow the subscriber during his movement in the cell.
The same modern technology enables one and the same antenna to have a plurality of steered beams, which are then directed in those directions where there at the moment are subscribers. It will be realised that if energy is only transmitted in directions where there are subscribers at the moment, this will permit energy to be saved. This "energy gain" can be used either to increase the range in those directions in which there is transmission, or to lower the output power of the antenna while maintaining the same range.
A common method of building steerable antennas is so-called group antennas. These are, as is indicated by the name, actually groups of antennas, often arranged in columns with several columns next to each other. Each separate antenna in such a column can consist of one antenna element, usually designed in so-called microstrip technology, which is excited by apertures in a ground plane. The apertures are arranged in groups, one for each antenna element, with one or several apertures in each aperture group, and are fed by means of a feeder network which is arranged in a further plane. The feeder network is also designed in microstrip technology. The feeder network may only cross the apertures in the connection points, the so-called feeding points. This means that the distance of the feeder network from the centre of the aperture groups to a great degree is decided by the extension of the apertures.
The feeder networks for the different columns may of course not cross each other either.
In order to avoid so-called grating lobes, i.e. lobes in undesired directions, the columns of the group antenna should be as closely positioned to each other as possible, especially in systems where one or several lobes are steered to a large angle relative to the normal of the antenna surface. The centre distance between the columns should be significantly less than one wavelength .lambda.; preferably it should be less than 0,5 .lambda..
Efficient design of group antennas, in other words, brings with it requirements for a compact antenna design, in which the feeder network can be arranged as close as possible to the centre of the aperture group.
In order to increase the availability of the system, so-called polarization diversity is often used, which means that each antenna in the group antenna is utilized in two directions of polarization. This, for example, makes it possible to receive signals which have had their polarization shifted as a result of reflections against surrounding objects, a phenomenon which can be particularly difficult in cities. In order to achieve a good isolation between the directions of polarization, it is extremely important that the antenna is symmetrical.
U.S. Pat. No. 4,903,033 shows a design for dual polarized antennas with a feeder network which, if two or several such antennas are to be connected to each other, can be said to require a great deal of space.
In "Proceedings of 16th ESA workshop on dual polarization antennas" there is on page 87, FIG. 13, a design for dual polarized antennas which permits a high degree of isolation between the directions of polarization, but if columns of two or several such antennas are to be connected it might be said that the distances between the feeder networks cause the columns to be placed farther apart than is desirable.
The object of the present invention is thus to obtain a dual polarized antenna intended to be part of a group antenna for wireless transmission of information using electromagnetic signals, which antenna is compact, has a high degree of symmetry, and permits the feeder network to be arranged closer to the centre of the aperture groups than previously.